Water-in-oil emulsions having a relatively high ratio of water phase to oil phase are known in the art as high internal phase emulsions ("HIPEs" or "HIPE" emulsions). Continuous processes for preparing HIPE emulsions are disclosed, for example, in Lissant, U.S. Pat. No. 3,565,817, issued Feb. 23, 1971 and Bradley et al, British Patent Application 2194166A, published Mar. 2, 1988.
The prior art has also recognized that the nature and characteristics of the porous polymeric foam materials formed by polymerizing HIPE emulsions is very much dependent on both the type of components which make up the polymerizable HIPE emulsion and the process conditions used to form the emulsion. For example, Unilever, European patent Application No. 60138, published Sep. 15, 1982 discloses a process for preparing absorbent porous polymers (i.e., foams) from high internal phase emulsions comprising at least 90% by weight of water with the oil phase containing polymerizable monomers, surfactant and a polymerization catalyst. Edwards et al, U.S. Pat. No. 4,788,225, issued Nov. 29, 1988 discloses the preparation of porous polymer materials which are rendered elastic by selecting certain monomer types (styrene, alkyl(meth)acrylates, crosslinker) and by using certain processing conditions to control the cell size of the eventually resulting porous polymer. Unilever, European Patent Application EP-A-299,762, published Jan. 18, 1989 discloses that the use of an electrolyte in the water phase of polymerizable HIPE emulsions can affect the size of the openings between cells of the eventually resulting porous polymeric foam material.
Notwithstanding the fact that the existence and synthesis of polymerizable HIPE emulsions is known in the art, preparation of HIPE emulsion suitable for polymerization to useful absorbent foam material is not without its difficulties. Such HIPE emulsions, and especially HIPE emulsions having a very high ratio of water phase to oil phase, tend to be unstable. Very slight variations in monomer/crosslinker content in the emulsion, emulsifier selection, emulsion component concentrations, and temperature and/or agitation conditions can cause such emulsions to "break" or to separate to at least some degree into their distinct water and oil phases. Even if stable emulsions can be realized, alterations in emulsion composition and processing conditions can significantly affect the properties and characteristics of the eventually realized polymeric foam materials, thereby rendering such foam materials either more or less useful for their intended purpose. Such HIPE emulsion preparation difficulties can become even more troublesome when there is a need to produce polymerizable emulsions via a continuous process on an industrial or pilot plant scale in order to provide commercially useful or development quantities of polymeric absorbent foam materials.
In present processes, HIPE is sampled from the process stream and viscosity is measured in a bench top couette viscometer.
It is, therefore, highly desirable to measure (or calculate) HIPE qualities in-line, i.e. without the need for sampling, whereby the whole fluid can be measured.
Applicant has found that viscosity is a useful process indicator of emulsion quality for the following reasons:
(1) Viscosity is highly sensitive to changes in the emulsion due to variations in process and formulation parameters.
(2) Viscosity is an intrinsic property of the emulsion that is independent of mixer hardware, so the data can be compared for different mixing systems.
(3) Viscosity can detect the presence of sweat (small amounts of free water) with high sensitivity.
(4) Viscosity provides immediate feedback whereas 24 hours or more may elapse before properties of the finished product can be measured.